What are the different memories used in a CISCO router?

Before we discuss the memories that are used in a Cisco router, let’s understand what basically a Router is.

When we want to connect two or more IP networks or subnetworks together, then a piece of hardware is needed and that hardware is called a Router. It generally serves two purposes:

1. Enabling the usage of a single Internet connection by several devices.
2. By forwarding packets to their designated IP addresses, traffic among these networks can be monitored.

PC1 and PC2 are connected to a router that is connected to Internet Service.

How does Router work?

Each data packet has a distinct destination and travels a distinct path to get there and these packets are guided by the router. The router also ensures that each data packet is successfully delivered to its destination without any interference in between the transmission. The best route is selected with the help of routing protocols like RIP(Routing Internet Protocol) /OSPF(Open Shortest Path First).

A routing table is used by the router internally where both the static (these types of addresses are assigned by the ISP(Internet Service Provider) and these types of addresses do not change) as well as dynamic addresses (these types of addresses are assigned using  DHCP(Dynamic host configuration Protocol) and these addresses change over time, they are not fixed) are stored.

Different types of Memories in a Router :

1. Random Access Memory (RAM) –
Random Access Memory (RAM) in a router is similar to the RAM that is installed in our PCs, Mobile Phones, and Laptops. The RAM is categorized into two areas:  

• Main Processor Memory, where information related to running router configuration (commands that we are currently running), routing table, and ARP (Address Resolution Protocol) cache is stored/saved.  
• Shared I/O Memory, which acts as a temporary storage memory where the data packets that are in queue are stored.  

Whenever the Router is rebooted or restarted, all the data, all the information that is stored in the RAM gets removed or deleted. The data gets deleted because all the information in the RAM is temporarily stored in a router. We can permanently store the data in NVRAM.

2. Non-Volatile Random Access Memory (NVRAM) :
NVRAM is used to store the startup configuration file. Startup configuration files are the copies of the Cisco Router Configuration file and they are retained after the router is restarted or rebooted. In NVRAM, if the router is rebooted or if the router is switched off then the data is not lost, it can be easily recovered. If we want to save the running configuration files permanently then we can move these files from RAM to NVRAM.

3. Read-Only Memory (ROM) :
A Cisco router’s boot procedure starts from the ROM memory section. ROM consists of programming instructions like POST (Power-On-Self-Test) and Bootstrap program. POST test is used to verify that whether hardware components like CPU, RAM, and interfaces are functioning properly or not. If they are not working properly then POST sends an error message. After this, the bootstrap application is used to set up the router’s CPU and boot functions. The bootstrap program is in charge of discovering and loading the router’s operating system (IOS). All this information is saved/stored in ROM and the data is retained even if the router is switched off or rebooted.

4. Flash Memory :
This is an Erasable Programmable Read-only Memory chip (EPROM). In flash memory, the operating system of the router I.e. IOS (Internetwork Operating System) is available. Unlike ROM, it is easily upgradable and does not require any hardware changes. In flash memory also, the content is retained whenever the router the switched off or rebooted.

Summarizing different memories that are used in Cisco Router – 

• RAM stores the currently working tasks. 
• NVRAM where startup configuration takes place. 
• ROM where the information of POST and bootstrap program is available. 
• And Flash Memory where the operating system of Router IOS is present.